1. Field of the Invention
This invention relates to improvements in methods and circuits for driving inductive loads, or the like, to which it is desired to apply signals having a fast rise time, and more particularly to improvements in methods and circuits for driving inductive loads of the type used in conjunction with a write head of a disk drives, or the like.
2. Relevant Background
Most computer systems include one or more associated disk drives, which may be built into the computer system, or which may be external to the computer system for connection via known means to the system. Typically, disk drives have at least one rotating magnetic medium and associated head mechanisms that are carried adjacent the material. The heads are radially positionable to selectively write information to, or read information from, precise positions on the disk medium. Such disk drives may be, for example, so-called hard disk drives, floppy drives, or the like.
Data is written to the associated data disk by applying a series of signals to the write head according to the digital information to be stored on the magnetic disk media. The write head has a coil and one or more associated pole pieces that are located in proximity to the disk media. As the magnetic flux changes in the head, the magnetic domains of the magnetic media of the disk are aligned in predetermined directions for subsequent read operations. Typically, a small space separates each magnetic domain transition to enable successive transitions on the magnetic media to be distinguished from each other.
Since the disk is moving relative to the head, it can be seen that if the small space separating the magnetic domain transitions is not sufficiently wide, difficulty may be encountered in distinguishing successive magnetic transitions. This may result in errors in reading the data contained on the disk, which is, of course, undesirable.
Meanwhile, as computers are becoming faster and faster, it is becoming increasingly important to increase the speed at which data can be written to the disk media. However, since the data signals are in the form of square wave transitions, if the rise time of the leading edges of the square waves is large, the small space between magnetic media transitions also becomes large, which reduces the effective rate at which data can be accurately written and read. Since the write head assembly includes at least one coil, forcing the current to rise rapidly, or to reverse flux directions, within the write head is difficult.
In the past, data writing circuits used to supply such write signals to the heads include preamplifier circuits to drive the current through selected legs of an "H-bridge" circuit, which is capable of allowing relatively fast current reversals for accurate data reproduction.
An example of a typical H-bridge write head data driving circuit 10, according to the prior art, is shown in FIG. 1. The circuit 10 includes four MOS transistors, 12-15 connected between a V.sub.CC rail 11 and ground 17. A coil 19, used, for example, to supply data pulses for writing to a disk drive media is integrated into the write head mechanism. It is connected between the center legs of the H-bridge, as shown.
It can be seen that, depending on the gate biases applied to the respective transistors 12-15, the current flows through the coil 19 in one direction or another. That is, one current flow path includes the transistor 14, coil 19 from right to left, and transistor 13. The other current flow path includes transistor 12, the coil 19 from left to right, and the transistor 15.
In the H-bridge circuit 10, the transistors 12 and 14 serve as switching transistors, which are controlled by the out-of-phase signals on the respective input lines 28 and 29. The transistors 13 and 15 serve as current control transistors, which are controlled by the out-of-phase signals on the respective input lines 29 and 28 in a manner opposite from the connections to the switching transistors 12 and 14, via respective control transistors 31 and 32. The magnitude of the current through the transistors 13 and 15 is controlled by transistor 21, with which the transistors 13 and 15 form respective current mirrors, when connected via respective transmission gates 24 and 25. The transmission gates 24 and 25 are controlled by the signals on the respective input lines 29 and 28, in the same manner as the associated transistors 31 and 32. A reference current source 26 supplies the reference current to transistor 21, which is mirrored by currents in respective transistors 13 and 15, as described above.
Thus, the data drive signals supplied to the head mechanism associated with the circuit 10 may be controlled by applying appropriate signals to the input lines 28 and 29. However, as mentioned, as data rates increase, the rates at which the heads can accurately write the data to the magnetic media is limited by the speed at which the flux in the coil 19 (and its associated components) can be reversed. The maximum data rate is thus limited to the maximum physical flux reversal rate of the driver circuitry.
It can be seen that in addition to the problem of reversing the flux in the coil 19 due to the inductive reactance of the coil, the time that is required to charge the gates of transistors 13 and 15 before they can conduct the proper current also results in a delay in reversing the flux in the coil 19. A waveform of a typical V.sub.GS seen at the gates and sources of transistors 13 and 15 is denoted by reference numeral 27. It can be seen that the waveform 27 has a significant rise time, due to the factors described above.
What is needed, therefore, is a method and circuit for driving an inductive load of the type used in conjunction with a write head of a disk drive for association with a computer or the like, with a signal that enables a maximum flux reversal rate in the driver coil.